Angelika Preetz

The Major/Minor Concept: Dependence of the Selectivity of Homogeneously Catalyzed Reactions on Reactivity Ratio and Concentration Ratio of the Intermediates

The homogeneously catalyzed asymmetric hydrogenation of prochiral olefins with cationic Rh(I) complexes is one of the best-understood selection processes. For some of the catalyst/substrate complexes, experimental proof points out the validation of the major/minor principle; the concentration-deficient minor substrate complex, which has very high reactivity, yields the excess enantiomer. As exemplified by the reaction system of [Rh(dipamp)(MeOH)2]+/methyl (Z)-alpha-acetamidocinnamate (dipamp=1,2-bis((o-methoxyphenyl)phenylphosphino)ethane), all six of the characteristic reaction rate constants have been previously identified. Recently, it was found that the major substrate complex can also yield the major enantiomer (lock-and-key principle). The differential equation system that results from the reaction sequence can be solved numerically for different hydrogen partial pressures by including the known equilibrium constants. The result displays the concentration-time dependence of all species that exist in the catalytic cycle. On the basis of the known constants as well as further experimental evidence, this work focuses on the examination of all principal possibilities resulting from the reaction sequence and leading to different results for the stereochemical outcome. From the simulation, the following conclusions can be drawn: 1) When an intermediate has extreme reactivity, its stationary concentration can become so small that it can no longer be the source of product selectivity; 2) in principle, the major/minor and lock-and-key principles can coexist depending on the applied pressure; 3) thermodynamically determined intermediate ratios can be completely converted under reaction conditions for a selection process; and 4) the increase in enantioselectivity with increasing hydrogen partial pressure, a phenomenon that is experimentally proven but theoretically far from being well-understood, can be explained by applying both the lock-and-key as well as the major/minor principle.

Trinuclear Rhodium Complexes and Their Relevance for Asymmetric Hydrogenation

Various trinuclear rhodium complexes of the type [Rh(3)(PP)(3)(mu(3)-OH)(x)(mu(3)-OMe)(2-x)]BF(4) (where PP = Me-DuPhos, dipamp, dppp, dppe; different ligands and mu-bridging anions) are presented, which are formed upon addition of bases such as NEt(3) to solvate complexes [Rh(PP)(solvent)(2)]BF(4). They were extensively characterized by X-ray diffraction and NMR spectroscopy ((103)Rh, (31)P, (13)C, (1)H). Their in situ formation resulting from basic additives (NEt(3)) or basic prochiral olefins (without addition of another base) can cause deactivation of the asymmetric hydrogenation. This effect can be reversed by means of acidic additives.

Halide bridged trinuclear rhodium complexes and their inhibiting influence on catalysis

The addition of halides to the cationic solvate complexes of the type [Rh(PP)(solvent)2][anion] leads to the formation of trinuclear μ3-halide-bridged complexes. The corresponding complexes [Rh3(PP)3(μ3-X)2][BF4] with X = Cl or Br and diphosphines PP Me-DuPHOS, Et-DuPHOS, DIPAMP, t-Bu-BisP* and Tangphos were characterized – in most cases – also by X-ray analysis. By reducing the concentration of the active catalyst, the in situ formation of these μ-halide-bridged multinuclear complexes in catalytic reactions leads to a decrease in activity or even to a total inactivity. The required halides – in most cases chloride – are usually present as impurities in the substrates (also when produced industrially). The extent of deactivation, known from enzyme catalysis as competitive inhibition, depends on several factors: the type of halide, the ratio of stability constants of multinuclear halide complexes and of substrate complexes, and the concentration of halide and substrate in solution.

Kinetics of Homogeneous Hydrogenations: Measurement and Interpretation

... By contrast, the measurement of the hydrogen consumption under normal pressure is relatively simple. The elementary structure of many such measuring devices is similar, and is based principally on the fact that the pressure drop is balanced by reduction in the reaction volume or by supply of the consumed gas, thus ensuring isobaric conditions. An appropriate device for monitoring major gas consumptions ...